Presently, a number of schemes exist to convert angular or linear mechanical position to a digital output. The most simple converters are potentiometers, which convert position to an output voltage by acting as a resistive divider. Their analog output can then be converted to a digital format, if required. In a case calling for an output signal that consists of discrete steps, a multiposition switch is the present choice.
With the advance of microprocessor-based equipment, digital incremental encoders have emerged. They typically employ a single track and two sensors arranged to produce a pair of square wave patterns with a 90.degree. phase shift. An up/down counter is connected to the sensor outputs to deliver position information. If a second track bearing an index marker is provided, the counter can be reset when the index position is sensed. Thus a quasi-absolute scheme results, in that accurate position information can be provided so long as power remains on, but after a power-up condition the position information is not accurate until a relative position is reached in which the index marker resets the counter.
Where true absolute information is required, for example where accurate position information is needed following a power-on condition without relative movement of the encoder parts, multitrack encoders are the most common choice. They typically feature a number of tracks equivalent to the .sup.2 log of the number of steps to be resolved, for example 8 tracks for a 256 step encoder. The aforementioned encoders most commonly utilize an optical scheme with a photographically fabricated coding wheel bearing a Gray code and LED/phototransistor combinations as detectors.
Since each track of a multitrack encoder contributes to the diameter of the device, it would be desirable, if possible, to reduce the number of tracks on the encoder without producing a corresponding reduction in the encoder's resolution. Although attempts have been made to design a Gray-coded absolute encoder that uses only a single track, these attempts have produced very inefficient encoders that utilize only a portion of the available output codes. For an n-sensor, binary, single-track (absolute) encoder, the best resolution that has heretofore been obtained is 4n (i.e., the number of output codes is equal to four times the number of sensors). See U.S. Pat. No. 5,029,304 to Tolmie, Jr., et. al. For a 5-sensor (or equivalently, five digit) binary encoder, for example, an encoder in accordance with the prior art would have a resolution of 4.times.5=20, and would thus utilize only 20 of the 2.sup.n =2.sup.5 =32 available output codes. This inefficiency becomes even more apparent as the number of sensors is increased. For a seven sensor binary encoder, for example, only 4.times.7=28 of the 2.sup.7 =128 output codes would be used.
This inefficiency in existing single track Gray-coded absolute encoders translates into unnecessary hardware. For example, for an application that requires a 128 position encoder, a single track binary encoder in accordance with the prior art would have 128/4=32 sensors and output lines, in comparison with 7 sensors and output lines for a conventional multitrack encoder. Thus, multitrack encoders remain as the preferred choice over single-track encoders, especially where high resolutions (e.g., 60 or more positions) are required.
Other principles worth mentioning are inductive or capacitive resolvers. Both schemes are by nature analog; their two or three phase outputs can be digitized and the rotary angle computed from the respective signal amplitudes. Synchro resolvers are a good example of a three-phase inductive rotary encoder. An application for a linear encoder is found in digital calipers.
One object of the invention is to provide an encoding principle which can yield a digitized absolute angular or linear position.
Another object of the invention is to produce a single track Gray-coded encoder having a resolution that approaches or is equal to the resolution of a mutitrack encoder, without a corresponding increase in the number of sensors.
Another object of the invention is to provide a single track encoder design that can be implemented using binary, ternary and quaternary sensing techniques.